1. Field of the Invention
The present invention relates to a field-effect transistor having an oxide film as a semiconductor layer, a method for manufacturing the field-effect transistor, and a display apparatus.
2. Description of the Related Art
In recent years, thin film transistors (TFTs) including a channel layer of a polycrystalline transparent conductive ZnO-based oxide thin film have actively been developed (see Japanese Patent Laid-Open No. 2002-76356).
Japanese Patent Laid-Open No. 2002-76356 stated that because the thin film can be formed at a low temperature and is transparent to visible light, a flexible transparent TFT can be formed on a substrate such as a plastic plate or a film.
However, a ZnO-based compound cannot form a stable amorphous phase at room temperature, but forms a polycrystalline phase. It is therefore difficult to increase electron mobility because of scattering at grain boundaries. Furthermore, the shape and the interconnection of polycrystalline grains vary widely with the film-forming method. This also produces variations in the characteristics of TFT devices.
Recently, a thin film transistor containing an In—Ga—Zn—O amorphous oxide has been reported (see K. Nomura et. al., Nature 432, 488 (2004)). This transistor can be formed on a plastic substrate or a glass substrate at room temperature. In addition, the transistor has a field-effect mobility approximately in the range of 6 to 9 cm2/Vs and normally-off characteristics. Furthermore, the transistor is transparent to visible light.
The present inventors studied TFTs containing oxides, including amorphous In—Ga—Zn—O, and found that the transistor characteristics of the TFTs sometimes varied, although the transistor characteristics depend on the composition and the manufacturing conditions of the TFTs.
The variations in transistor characteristics can cause variations in operation of organic light-emitting diodes (LEDs) and liquid crystals driven by the transistors, for example, in a pixel circuit of a display.
The variations may be caused by parasitic resistance generated between a source electrode and a channel and between a drain electrode and a channel.